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Prevent unwanted halos when moving image selections

Added on Friday 18th of September 2009 08:22 am EST

Application:

Adobe Photoshop CS/CS2/CS3/CS4

Operating System:

Macintosh, Microsoft Windows

Pasting selected image areas from one photo to another is a simple process when using Photoshop’s Anti-aliased command. But often the pasted image displays an unwanted and unsightly light-colored ring or halo around its perimeter. By making use of the right selection technique, you can avoid halos for seamless results every time.

To show you how to avoid unwanted halos, we’ll:

•Identify what causes halos so you understand why they appear when pasting an object.

•Go over the various ways to prevent halos so that you’re aware of what choices you have to work with.

•Demonstrate a technique showing how you can avoid halos in your pasted images.

The Anti-aliased command, which smoothes the ragged edges of your selection, is common to all of the selection tools. But often when you paste an anti-aliased selection, some of the pixels surrounding the selection border become part of the selection. This results in an unsightly fringe, or halo, as shown in Figure A. Erasing the halo using the Eraser tool would take too much time and effort. But by using a combination of the Modify command and the Feather command, you can easily create a selection that’s both smooth-edged and halo free, as shown in Figure B. Let’s start by examining how the selection process works.

A

B

Understand the selection process

When you make an image area selection, you can choose an area as small as a single pixel or as large as the entire image. Once you’ve made your selection, a selection border surrounds the chosen pixels, as shown in Figure C. The selection border shows you what you’ve selected. The pixel area outside a selection border is protected from edits, while the area within the selection border isn’t protected and is available for editing.

C

Create hard- and soft-edge selections

Initially, the selection border creates a hard-edge transition between what you’ve selected and what you haven’t.

When the edge between the selection and its background is clearly defined, as shown in Figure D, a hard-edge selection causes very few problems when you paste the selected pixel area into a background, as shown in Figure E.

D

E

When the edge isn’t as clearly defined however, as in Figure F, a hard-edge selection causes a problem, as shown in Figure G. This is because there’s no well-defined line of separation that you can use to form the natural shape of the selected pixel area.

F

G

Apply the anti-aliasing option

To smooth out the rough edge of the selected pixel area, apply the Anti-aliased option to the edge. Anti-aliasing is available for all selection tools except the Rectangular, Single Row, and Single Column Marquee tools. Anti-aliasing smoothes the hard edge by blending the color transition between the edge pixels and the adjacent background pixels. In creating the color transition, however, anti-aliasing includes a few pixels of the adjacent background beyond the selection border. In doing so, a halo is often created as a side effect.

Prevent halos with the Modify and Feather commands

To prevent anti-aliasing from including or expanding to adjacent pixels, you have to decrease the selection border slightly before applying the Anti-aliased option. To do this, use the Modify command, which adjusts the selection border’s dimensions by numerically decreasing the amount of pixels included in an existing pixel area selection.

However, even when you do decrease the selection border slightly and apply anti-aliasing, you still might find the edge transition a little too hard. To soften the edge further, you can feather the edge using the Feather command. Feathering softens a hard edge by creating a transition boundary between the selection edge and its surrounding pixels.

Let’s now use the Modify and Feather commands to demonstrate how to prevent halos from occurring in an anti-aliased selection.